1. Field of the Invention
The present invention relates generally to a functional tunable multichannel filter that includes one or more polarization maintaining fiber filters, polarization controllers (λ/2, λ/4) and a 3 dB coupler and, more particularly, to an independent functional tunable multichannel filter that is not influenced by the varying polarization of an input signal because it uses one or more polarization maintaining fiber filters, polarization controllers (λ/2, λ/4) and a 3 dB coupler.
2. Description of the Related Art
Existing multichannel filters include a fiber optic grating filter, a Mach-Zehnder interferometer and an arrayed waveguide grating. In the case of the optical fiber grating filter, a multichannel filter is implemented by connecting a plurality of gratings corresponding to respective wavelengths. Accordingly, the optical fiber grating, filter is disadvantageous in that the cost thereof is high and each grating filter must be tuned to achieve wavelength tuning. Although a wavelength position can vary by tuning each individual grating filter, it is difficult to adjust uniform channel spacing. Furthermore, when the optical fiber grating filter is commercialized, a problem occurs in that its size increases.
The Mach-Zehnder interferometer has multichannel filter characteristics when exact coincidence in variation in phase is achieved. However, the Mach-Zehnder interferometer is disadvantageous in that it is difficult to ensure stable performance because the coincidence in variation in phase is very sensitive to variation in outside temperature, vibration, strain or the like. Furthermore, the Mach-Zehnder interferometer is disadvantageous in that it is difficult to control a wavelength position and channel spacing.
Furthermore, the arrayed waveguide grating is disadvantageous in that the phase of each arrayed waveguide must be exactly adjusted, it is difficult to select a desired channel, and its size increases when commercialized. Although a wavelength position can easily vary with temperature, a problem occurs in that it is difficult to adjust channel spacing. Furthermore, the three conventional multichannel filters described above are sensitive to the varying polarization of an input signal and, therefore, it is necessary to reduce dependence on polarization.